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A Pinch of Science

THE New York City Department of Health and Mental Hygiene, a leader in promoting public health, has embarked on a campaign to persuade the makers of processed food to reduce its salt content by more than 40 percent over the next 10 years. The goal is commendable: to prevent strokes and heart attacks. And the premise is logical: if people eat less salt, they’ll have lower blood pressure, and this could translate into better cardiovascular health.

If such a large reduction were actually to be achieved, however, New Yorkers would consume less sodium than people in most other developed countries do. And there is a possibility that such a big change in one element of their diet might have unintended harmful consequences. Prudence requires that logic and good intentions also be supported by strong evidence that such an action would be safe.

Throughout history, efforts have been made to reform the human diet by changing individual characteristics of it, and some of these changes have had unexpected harmful effects. In the 1950s, for instance, pregnant women were urged to strictly limit their weight gain to avoid pre-eclampsia, a syndrome characterized by high blood pressure, fluid retention and kidney problems. Enough women apparently followed this advice that the number of underweight babies — and of infant deaths, some attributable to low birth weight — increased.

More recently, the federal Dietary Guidelines have been criticized by medical researchers as contributing to an increasing prevalence of obesity in the United States, in part by encouraging people to eat too much low-fat food.

In both instances, respected authorities instituted reasonable ideas without having the evidence to know whether their policies might backfire.

Salt — sodium chloride — is only one of many essential elements in a sound diet. In places where populations have free access to salt, healthy people typically consume about five to eight grams (giving them two to four grams of sodium) per day. When groups of people reduce their daily sodium intake by one to two grams, their average blood pressure falls. But there is tremendous variation among individuals. For most people, wide swings in dietary sodium consumption don’t affect blood pressure, and for some, blood pressure actually rises when they lower their salt intake.

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Scott Menchin

But what really matters is whether reducing salt will ultimately prevent heart attacks and strokes and thus improve or extend life. And this depends not on blood pressure alone, but on all the metabolic consequences of limiting dietary sodium — which include greater resistance to insulin, increased sympathetic nerve activity and activation of the kidney-based renin-angiotensin system. All three of these effects increase the risk of heart attack and stroke. It is also possible that a change in this single dietary element might disturb unknown nutritional interactions and thus generate other as yet unrecognized effects, good or bad.

The best available evidence on how salt consumption affects our health comes from observational studies, in which groups of subjects are investigated to identify any correlations between usual sodium intake and subsequent heart attacks and strokes. Nine such studies, looking at a total of more than 100,000 participants who consume as much sodium as New Yorkers do, have had mixed results. In four of them, reduced dietary salt was associated with an increased incidence of death and disability from heart attacks and strokes. In one that focused on obese people, more salt was associated with increased cardiovascular mortality. And in the remaining four, no association between salt and health was seen.

People who advocate curtailing salt consumption typically prefer to discuss two other observational studies from Finland and Japan, where salt consumption is generally higher than in the United States. In both of these, more salt was associated with more cardiovascular problems.

But observational studies do not demonstrate causality. And this kind of research can justify action only when multiple studies produce consistent, robust findings across a wide range of circumstances, as the research on tobacco and lung and cardiovascular health has done.

Nevertheless, the research on salt intake can help identify questions to address in randomized clinical trials, the most rigorous kind of medical research. In such a study, people who share relevant characteristics like age, weight, blood pressure and cholesterol levels are divided into two groups, which follow the same diet in every way except that those in one group would reduce their salt intake. This way any differences in outcomes (heart attacks and strokes) can be reliably attributed to sodium consumption.

Only one such rigorous clinical trial on salt intake has been reported so far, and it focused on patients with fairly advanced heart problems. As it turned out, the group that adhered to a lower sodium diet actually suffered significantly more cardiovascular deaths and hospitalizations than did the one assigned to the higher sodium diet.

Another bit of evidence to consider is that, over the past generation, while sodium intake in the United States appears to be increasing, deaths from heart attacks and strokes have declined by half.

Absent convincing scientific evidence of a benefit to eating less salt, much less an assurance of safety, it might be wiser for the New York City Health Department to press for the research that could provide a solid scientific basis for action. And in the meantime, to help people lower their risk of heart attack and stroke, health officials should concentrate on promoting the benefits of weight control (which reduces salt intake) and physical exercise.

Michael Alderman is a professor of medicine and epidemiology at Albert Einstein College of Medicine.

A version of this op-ed appears in print on , on page A27 of the New York edition with the headline: A Pinch of Science. Today's Paper|Subscribe